Cellular Kinetics of Inflammation in the Pleural Space of Mice in Response to the Injection of Exogenous Particles

Abstract

CD-1 mice were used to study the cellular kinetics of the inflammatory response of the pleural space to the injection of 250 μg of silica or of tungsten microparticles. The pleural exudates were collected by lavage of the serous cavity of mice that were sacrificed at 30 min and up to 7 days after the intrapleural instillation of the particles. The samples were studied by light and electron microscopy (transmission and scanning modes); the quantitative cellular kinetics of the inflammation was determined by leukocyte counting in exudates using cytocentrifuge preparations. The normal resident population of cells of CD-1 mice was made up of (2.47 ± 0.37) × 10⁶ cells. It consisted mostly of macrophage-like cells ((2.03 ± 0.26) × 10⁶ cells, 82% of total cells), some lymphocytes ((0.37 ± 0.07) × 10⁶ cells, 15% of total cells), a few mast cells and eosinophilic granulocytes (1–2% of total cells). The initial inflammatory reaction (30–60 min after injection) was characterized by a decrease in the number of cells harvested from the pleural space. This was followed by an intense recruitment of granulocytes and monocytes that resulted in a peak of intrapleural cells at 24 h ((16.8 ± 4,0) × 10⁶ cells induced by silica particles and (18.3 ± 4.2) × 10⁶ cells induced by tungsten particles). In tungsten-injected mice (but not in silica-treated animals) the enhancement in the number of intrapleural macrophages continued up to 72 h after particle injection. The highest percentage of macrophages with ingested tungsten (50% of total macrophages) was found early (6 h) and decreased thereafter; at day 7 it encompassed just 17% of the macrophages. Injection of any of the two particulates led to the disappearance of mast cells from the pleural space of mice. Silica particles attracted a high number of eosinophils to the pleural cavity of mice. Light and electron microscopy documented that pleural macrophages underwent striking morphological changes during the inflammatory response: the phagocytes showed marked increase in size and in number of surface processes, and their cytoplasm often contained large amounts of the injected particles and also of cellular debris. This study establishes the mouse as a reliable animal model to study the dynamics of the pleural space and it offers a precise definition of the cellular kinetics of inflammation in this serous cavity. The data indicate that the kinetics of experimental pleural inflammation induced by particulates may depend on the nature of the injected particles.